The simple past is employed here because you are referring to historical events, since the rest of the sentence mentioned the 80s. So it is a completed action in the past. The present perfect is usually employed to describe actions that happened in the past but at no specific point in time. It is also employed when an action starts in the past and is still continuing in the present or when an action has recently finished (with a potential impact on the present). I hope that helps

Kept for

Of the three possibilities: "has kept", "had kept", and "kept", only "has kept" is wrong (for the reasons explained by Pierre). "Kept" and "had kept" give the same meaning but differ slightly in emphasis. When you say "The fur trade had kept ... but when the market collapsed ..." you give emphasis to the change that occurred in the 1980s. The market had been one way before the 1980s, and then it changed. When you say "The fur trade kept ... but when the market collapsed ..." you are placing less emphasis on the change. Instead, you are saying the market was one way before the 1980s, and it was another way afterwards. This point of view describes history without drawing particular attention to the change that occurred in the 1980s.

It should be written as "had kept". Past perfect.